The invention relates to a process for preparing pure TEOA by continuously distillatively separating an ethanolamine mixture comprising TEOA and diethanolamine (DEOA), by distilling off DEOA in a distillation column (DEOA column) and supplying the resulting bottom stream comprising TEOA to a downstream column (TEOA column) in which the pure TEOA is withdrawn as a side draw stream.
It is common knowledge that an initially colorless, pure TEOA obtained after a fractional distillation of a crude TEOA product which has been obtained by reacting aqueous ammonia with ethylene oxide and distilling off monoethanolamine (MEOA) and diethanolamine (DEOA) (color number: approx. 0 to 20 APHA to DIN-ISO 6271(=Hazen)), after a storage time of approx. 4 to 6 weeks, even in a closed container and with exclusion of light, can gradually turn pale pink and finally, particularly readily when standing under light, yellow to brown. This effect is accelerated by the action of relatively high temperatures (see, for example: G. G. Smirnova et al., J. of Applied Chemistry of the USSR 61, pages 1508-9 (1988), and Chemical & Engineering News 1996, Sep. 16, page 42, middle column).
According to Chemical & Engineering News 1996, Sep. 16, page 42, one mole of TEOA decomposes to one mole of ethanolamine and 2 mol of acetaldehyde at elevated temperature. Acetaldehyde condenses to crotonaldehyde, which in turn forms a Schiff base with ethanolamine. This unsaturated Schiff base leads, with 1,4-polymerization, to colored products in the TEOA.
A method of assessing the color quality of pure TEOA which has been found to be useful, in addition to the time-consuming storage tests in which the APHA color number (to DIN-ISO 6271) of the TEOA is measured as a function of the storage time, is the so-called acid neutralization test.
This acid neutralization test allows the assessment of the color stability in the course of storage of freshly prepared TEOA within a few minutes.
The test is described in Japanese documents JP 62 019 558 A (Derwent abstract No. 87-067647/10) and JP 62 005 939 A (Derwent abstract No. 87-047397/07), according to which the TEOA is treated (neutralized) with acetic acid, citric acid, sulfuric acid, hydrochloric acid or phosphoric acid, and then the absorbance of the absorption bands is measured at 420 nm and 530 nm. When no apparent pink coloration of the TEOA occurs during the performance of the test and the values measured for the absorbance remain sufficiently low, the TEOA is color-stable in the course of storage, i.e. remains colorless over a period of several months,
The literature describes various methods of preparing pure and colorless to low-color TEOA.
EP 4015 A (BASF AG) states that ethanolamines with relatively low discoloration are obtained by adding phosphorous acid or hypophosphorous acid during the preparation of the ethanolamines and/or the distillative workup.
EP 36 152 A and EP 4015 A (both BASF AG) explain the influence of the materials used in processes for preparing alkanolamines on the color quality of the process products and recommend nickel-free or low-nickel steels.
U.S. Pat. No. 3,819,710 discloses a process for improving the color quality of ethanolamines by hydrogenating the crude ethanolamines in the presence of selected catalysts. However, the process is technically complex and does not lead to a TEOA product which remains colorless over several months.
U.S. Pat. No. 3,207,790 describes a process for improving the color quality of alkanolamines by adding a borohydride of an alkali metal.
U.S. Pat. No. 3,742,059 and DE 22 25 015 A describe the improvement of the color quality of alkanolamines by the addition of an alkanolamine ester of boric acid or alkali metal/alkaline earth metal borates.
The presence of an assistant to stabilize TEOA is, however, undesired in many important areas of use of the TEOA.
The subsequent addition of small amounts of ethylene oxide to freshly distilled TEOA leads, according to U.S. Pat. No. 4,673,762, likewise to decoloration and color stabilization, However, the method appears to be hazardous for toxicological reasons.
GB 1 062 730 A describes a process for purifying ethanolamines by purifying distillation in the presence of silicates or aluminates.
JP 62 019 558 A (Derwent abstract No. 87-067647/10) reports the preparation of qualitatively good TEOA by treatment of crude TEOA with inorganic oxides at 170 to 250° C. and subsequent distillation in the absence of oxygen.
Similar results are achieved according to JP 62 005 939 A (Derwent abstract No. 87-047397/07) when crude TEOA is heated to 170 to 250° C. with exclusion of air for 1 to 10 h and then distilled under reduced pressure.
SU 326 178 A (Derwent abstract No. 63384T-AE) describes the preparation of TEOA with good color quality by gentle reaction of anhydrous monoethanolamine (MEOA) or diethanolamine (DEOA) or mixtures of the two substances with ethylene oxide at temperatures less than 50° C.
Similar results are achieved according to SU 228 693 A (Chem. Abstr. 70, 77305f (1969)) and GB 1 092 449 A when ammonia is reacted with ethylene oxide at less than/equal to 35° C. and the resulting ethanol amine mixture is distilled with exclusion of air.
From an economic point of view, those processes in which the reactions with ethylene oxide take place at low temperatures are inefficient owing to the long residence times and the associated low space-time yields.
WO 2001/53250 A1 (BP Chemicals Ltd.) relates to a continuous process for the manufacture of TEOA comprising (i) a step of synthesizing the TEOA by continuously bringing ammonia into contact with ethylene oxide, under conditions allowing the formation of a reaction mixture comprising mono-, di- and triethanolamines, (ii) a step of continuously separating the ammonia that has not reacted from the reaction mixture, and (iii) a step of continuously separating the TEOA from the mixture resulting from step (ii), which process is characterized in that, in the last step, the monoethanelamine and some of the diethanolamine are separated from the mixture resulting from step (ii), a specific mixture of alkanolamines comprising TEOA and from 0.5 to 50% by weight of at least one secondary dialkanolamine is prepared or isolated, and in that the TEOA is separated and isolated with a degree of purity of ≧99.2% by weight by continuous distillation of the specific mixture of alkanolamines.
WO 2005/035481 A2 (BASF AG) describes the distillative removal of triethanolamine, in which the substance mixture is distilled in two stages. In the first stage, the low boiler fraction and the high boiler fraction are withdrawn and discharged, and, in the second stage, the medium boiler fraction with a triethanolamine content of >99.4% by weight and a diethanolamine content of <0.2% by weight is distilled. The substance mixture is distilled preferably in a first column and a second column connected thereto, or in a dividing wall column.